With a view to a better understanding, it should be reminded that:
during an accidental or chronic discharge (for example pipeline breakage, tightness loss of a storage means, for example a vat or a tank) or after the shutdown of old industrial sites, a certain amount of hydrocarbon compounds can infiltrate into soils, which leads to the pollution of all or part of said soil, PA1 knowledge of the pollutant type (gasoline, kerosene, gas oil, lubricant, chlorine derivatives, etc.) as well as knowledge of the extension in space and in time of the pollution is of great significance for people in charge of diagnosis studies, environmental impact studies and polluted soil rehabilitation. It is in fact well known that the rehabilitation techniques used depend on the type of pollutants. PA1 a) the temperature of the sample is quickly raised to a first temperature value T.sub.1 ranging between 80 and 120.degree. C. for a determined period (t.sub.1 -t.sub.0), PA1 b) from the first temperature value, the temperature of the sample is raised to a second temperature value T.sub.2 below 200.degree. C. according to a temperature gradient ranging between 2 and 30.degree. C./min, and this temperature T.sub.2 is maintained for a determined period (t.sub.3 -t.sub.2), PA1 c) the temperature is raised from the second value to a third temperature value T.sub.3 below 500.degree. C., according to a temperature gradient ranging between 10 and 40.degree. C./min, PA1 d) the temperature of the sample is raised from the third value to a fourth value T.sub.4 at most equal to 850.degree. C., PA1 e) at least four quantities Q.sub.0, Q.sub.1, Q.sub.2, Q.sub.3 representative of the nature and of the amount of hydrocarbon compounds contained in said sample are determined from the previous four stages, PA1 f) after raising the temperature to the fourth value T.sub.4, the residues of said sample are burned in an oxidizing atmosphere from a temperature above 350.degree. C. to a temperature at most equal to 850.degree. C., according to a temperature gradient ranging between 20 and 50.degree. C./min, PA1 g) a quantity Q.sub.4 representative of the amount of residual organic carbon after the four heating stages is determined, PA1 h) at least one pollution characteristic of said sample is deduced from the five quantities Q.sub.0, Q.sub.1, Q.sub.2, Q.sub.3 and Q.sub.4. PA1 for a pollutant of type "a", the ratio Q.sub.0 /(Q.sub.0 +Q.sub.1 +Q.sub.2) ranges between 0.5 and 1 and the ratio (Q.sub.3 +Q.sub.4)/(Q.sub.0 +Q.sub.1 +Q.sub.2) is close to zero, PA1 for a pollutant of type "b", the ratio Q.sub.0 /(Q.sub.0 +Q.sub.1 +Q.sub.2) ranges between 0.05 and 0.5 and the ratio (Q.sub.3 +Q.sub.4)/(Q.sub.0 +Q.sub.1 +Q.sub.2) is close to zero, PA1 for a pollutant of type "c", the ratio Q.sub.0 /(Q.sub.0 +Q.sub.1 +Q.sub.2) is close to zero, and the ratio (Q.sub.3 +Q.sub.4)/(Q.sub.0 +Q.sub.1 +Q.sub.2) ranges between 0 and 5, PA1 for a pollutant of type "d", the ratio Q.sub.0 /(Q.sub.0 +Q.sub.1 +Q.sub.2) is close to zero, and the ratio (Q.sub.3 +Q.sub.4)/(Q.sub.0 +Q.sub.1 +Q.sub.2) is above 10.
It is therefore very important to be able to determine quickly the nature of the pollutants and the amount of soil to be treated, that directly depends on the extension of the pollution in depth as well as at the surface. Determination of the degree of pollution of a soil allows to evaluate the volumes of ground to be treated, to determine the best treating methods and thus the costs corresponding to the implementation thereof.
It thus appears that systematic analysis of samples of potentially or really polluted soils allows to make quickly a diagnosis concerning the nature and the extent of the pollution, as well as the main associated risks (contamination of a groundwater table for example).
Knowledge of such information allows to optimize operations of rehabilitation of contaminated sites from the diagnosis stage onwards. Long and costly laboratory analyses, without being completely suppressed, are limited to the necessary minimum amount complementing the information systematically collected by means of the method according to the present invention.
The ROCK-EVAL method developed by the claimant is notably described in documents U.S. Pat. Nos. 4,153,415; 4,229,181; 4,352,673; 4,519,983, as well as French patent application FR-94/08,383. This method, which is fast, practically automatic, has been developed for characterizing mother rocks or reservoir rocks and the hydrocarbons they contain.
However, this method and the device are not suited to characterize precisely the various hydrocarbon cuts that can be contained in soils polluted by such products.
The present invention is thus an improvement on the ROCK-EVAL technique allowing to characterize precisely the pollutants, notably of the hydrocarbon type and/or derivatives (chlorine, sulphur compounds, . . . ), contained in a polluted soil.